The Echorium Sequence - Katherine Roberts

This trilogy for children comprises of Song Quest, Crystal Mask and Dark Quetzal. The series tells of a Dark Lord, whose name is Frazhin, but he styles himself Khizpriest, Starmaker, and Lord of the Forest at various times in the trilogy. He is out to destroy the Singers who live on the Isle of Echoes, 30 days sailing from the mainland. The Singers use five special wordless Songs of Power (which they hum or sing) to affect people's mood and behaviour:

Challa is the Dream Song and is used for healing. It sends people to sleep and helps the forget their troubles.
Kashe is the Laughter Song and is used to wake people up and cure depression.
Shi is the Pain Song and it is used to confront people's pain and heal through tears.
Aushan is the Fear Song and it is used to give life to inner fears. It generally makes people scream.
Yehn is the Death Song and it closes doors in the head (ie. makes them forget things). In extreme cases it is used to remove someone's soul, leaving them alive but with no will or memories of their own.

Frazhin uses the khiz-spear to control the people of the Karch, a mountainous region on the mainland. He wants to destroy all Singers and change history so that they will never have existed. He tries to control the Karchholders by feeding their Karchlord poisoned Merlee (merpeople) eggs. He also tries to control the Horselords of the Purple Plains, and he corrupts many of the Half Creatures (Merlee who are part human, part fish; Naga who are part human, part water-snake; Quetzal who are part human, part bird and brilliant mimics; and Centaurs who are part human, part horse). At various times throughout the trilogy, fully trained Singers (who besides knowing the Songs of Power, also have the skills of truth listening - the ability to tell when someone is lying by listening carefully and reading their body language - and farlistening - listening for vibrations over a greater distance than is normally possible for the human ear) and several child novices, who are about to become Singers themselves (so are in their final year of training), become involved in the battles to prevent Frazhin from achieving his goals. Rialle befriends the Merlee, Renn befriends a Centaur, and Caell befriends a Quetzal (although all three of them can understand all Half Creatures and converse with them in their own wild speech).

In each book of the trilogy a Singer goes to the mainland with one of the novices and one or more other young people to try to prevent Frazhin from achieving his aims. In each case they are victorious but they do not kill Frazhin or overcome him finally (eg. by giving him Yehn), although he is wounded. 20 years pass between the events of Song Quest and Crystal Mask, and 11 years pass between the events of Crystal Mask and Dark Quetzal, so the three novices grow up, and need a novice to assist them in talking to the Half Creatures in the later books, since most adult Singers lose the ability to hear wild speech as they age.

The trilogy is interesting, although slightly repetitive because of the recurrent threat from Frazhin, but the changing cast of younger characters, not to mention the fact that those younger characters recur as adult characters, keeps the reader's interest and prevents the books from being boring. The fact that different races of Half Creatures are introduced in each book also adds interest to the stories.